Research on selenium toxicity has been in progress for over four decades and the essentiality of the element has been studied for over one decade, yet there is relatively little known of the organic forms of selenium within animals. Our laboratory has identified trimethylselenonium ion as an excretory product of selenium metabolism in rat urine and we have recognized the presence of another major metabolite. It is our objective to isolate and identify the unknown metabolite and to determine its toxicity and biological activity in rats. Selenium-75 will be used to facilitate the fractionations procedures. Since very little is known concerning the steps involved in the reduction of inorganic selenium and its incorporation into organic compounds, studies will be undertaken to elucidate the bio-synthetic pathway for the new metabolite and the previously identified methyl derivatives of selenium. This will be accomplished by injecting rats with various proposed intermediates such as those suggested by Challenger as being involved in the methylation of selenium by molds. From the end products produced, it should be possible to determine the sequence of reactions involved in selenium reduction and methylation. Further work will involve determining whether the new metabolite and the previously identified methylated metabolite are peculiar to rats or if they are common excretory forms in other species.